The belligerents during World War II fought as partners in one of two major alliances: the Axis and the Allies. The three principal partners in the Axis alliance were Germany, Italy, and Japan. The Axis partners had two common interests: 1) territorial expansion and foundation of empires based on military conquest and the overthrow of the post-World War I international order; and 2) the destruction or neutralization of Soviet Communism.

On November 1, 1936, Germany and Italy announced a Rome-Berlin Axis one week after signing a treaty of friendship. On November 25, 1936, Nazi Germany and Imperial Japan signed the so-called Anti-Comintern Pact directed at the Soviet Union. Italy joined the Anti-Comintern Pact on November 6, 1937. On May 22, 1939, Germany and Italy signed the so-called Pact of Steel, formalizing the Axis alliance with military provisions. Finally, on September 27, 1940, Germany, Italy, and Japan signed the Tripartite Pact, which became known as the Axis alliance.

Hungary joined the Axis on November 20, 1940 and Romania joined on November 23, 1940. Both politically and economically dependent on Germany for its very existence as an “independent” state, Slovakia followed suit on November 24. Bulgaria joined the Axis on March 1, 1941 and Yugoslavia on March 25, 1941. Two days later, Serbian military officers overthrew the government that had signed the Tripartite Pact. After the subsequent invasion and dismemberment of Yugoslavia by Axis forces in April, the newly established and so-called Independent State of Croatia joined the Axis on June 15, 1941. On June 26, 1941, four days after the Axis invasion of the Soviet Union, Finland entered the war against the USSR as a “co-belligerent.” Finland never signed the Tripartite Pact.

The Allied Powers, led by Great Britain, the United States, and the Soviet Union, defeated the Axis in 1945.